One or more embodiments relate generally to the fields of image enhancement and video post-processing. More particularly, one or more of the embodiments of the invention relates to a method and apparatus for hardware-based video/image post-processing.
One-dimensional video scaling provides a technique for either increasing or decreasing the number of pixels in a frame. For example, upscaling may increase the number of pixels in a frame, whereas downscaling may decrease the number of pixels in a frame. Upscaling may be used to enable display of a 16:9 widescreen format to, for example, a 4:3 standard television format. In other words, to fit the widescreen format onto the taller 4:3 format of a television, “vertical” upscaling may be performed to stretch the widescreen format in a vertical direction to occupy the entire television screen. Likewise, “horizontal” scaling may be used to stretch a television format in a horizontal direction to occupy a widescreen television.
One problem with upscaling is that the resulting picture shown on the television screen may be flawed since the items on the screen are stretched, or upscaled, in one direction and not correspondingly scaled in another direction. To avoid the flaws of purely one-dimensional scaling, another technique for performing scaling is referred to as anamorphic scaling. In addition to processing in two dimensions, anamorphic scaling generally requires an non-integral scaling factor. One technique for implementing anamorphic scaling is the use of hardware-based polyphase filters. For example, upscaling may be used to fit a 1280×720 pixel input onto a 1920×1080 output screen. In this example, the image is upscaled vertically and horizontally by the same factor i.e., 1.5 times. However, in some cases the image might need to be upscaled by a different amount vertically and horizontally. For example, to enable viewing widescreen (16:9) input onto a 4:3 TV, the image might need to be stretched vertically to fit the taller 4:3 format of the TV. Likewise, extra horizontal stretching might be required when trying to show 4:3 content on a 16:9 TV.
One problem with scaling the image differently vertically and horizontally, is that the resulting picture shown on the television screen may be flawed since the items on the screen are stretched, or upscaled, more in one direction and scaled less (or not at all) in another direction. To avoid the flaw, a different scaling algorithm is used which is anamorphic scaling. Anamorphic scaling is still 1-D scaling, however different portions of the screen are scaled by different amounts. As an example, to fit 4:3 content onto a 16:9 content, instead of stretching the picture horizontally by the same amount everywhere, the center of the screen is not stretched (so that objects in the center look natural), and the picture edges are stretched more (to fill the screen).
Typically, 1-D image scaling is done using polyphase filters. Unfortunately, implementing anamorphic scaling in hardware-based polyphase filters is typically a very costly process because the scaling factor does not remain constant. Generally, high-end polyphase filters often have up to 64 different coefficient sets comprised of multiple coefficient values. In a worst case scenario, anamorphic scaling requires a different scaling factor for every pixel. In addition, each different scaling factor requires a different coefficient set to compute the output pixel. Hence, a unique coefficient set may be required to compute each output pixel. In other words, to scale a row of 1920 pixels (high-definition (HD) content), 1920 different coefficient sets (for the worst case scenario) are required. Consequently, the 64 different coefficient sets provided by a high-end polyphase filter are generally inadequate for performing scaling of HD content.